<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<style>

		body {
			padding: 0;
			margin: 0;
		}

		.box {
			width: 600px;
			height: 400px;
			margin: 100px auto;
			border: 1px solid #CCC;

			display: flex;
		}

		.inner {
			width: 200px;
			height: 200px;
			background: red;

			margin: auto;
		}
	</style>
</head>
<body>
	<div class="box">
		<div class="inner"></div>
	</div>
</body>
</html>